<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<select  id="s1" onchange="changeCity()">
    <option value="中国">中国</option>
    <option value="美国">美国</option>
    <option value="法国">法国</option>
</select>
<select  id="s2">
    <option>北京</option>
</select>
<script>
    function changeCity() {
        if(s1.value=="中国"){
            s2.innerHTML="<option>北京</option>"
        }else if(s1.value=="美国"){
            s2.innerHTML="<option>华盛顿</option>"
        }else if(s1.value=="法国"){
            s2.innerHTML="<option>巴黎</option>"
        }
    }
</script>
</body>
</html>